The present invention relates generally to optical networks and, more particularly, to a method and apparatus for automated time domain monitoring in optical networks.
Generally, digital transmission systems deliver information, which is encoded as quantized signals, from a sender to a receiver. There are several parameters that characterize the quality of the transmission in these systems. One such parameter is the xe2x80x9cbit error ratioxe2x80x9d or BER. BER can be measured at any point in a transmission system, and may be used for fault detection and isolation.
There are many effects that contribute to degradation of a measured BER. For example, signal attenuation reduces signal amplitude, dispersion alters pulse shape, receiver and amplifier noise increase signal level ambiguity, and jitter creates uncertainty in the sampling point and affects other aspects of synchronization.
In an optical network, such as a Wavelength Division Multiplexing (WDM) network, data passes through many different types of network elements: wavelength converters and filters, wavelength add-drop multiplexer (ADM), cross-connects, and optical amplifiers. The network elements may perform multiple optical to electronic (O/E) and electronic to optical (E/O) conversions, or alternatively, may process the data in optical form. Although it is preferable to perform signal monitoring at network elements that include O/E and E/O converters, one can also tap and detect a signal at any point in a WDM network.
FIG. 1 illustrates a prior art optical signal monitoring system 100, which monitors optical signals on a fiber link by performing indirect SNR or average power level measurement. As shown, optical monitoring system 100 includes an optical fiber 110 carrying an input optical signal, optical power splitter 120, optical fiber 130 carrying output optical signal, wavelength selective filter 140, photodetector 150, electrical amplifier 160, and a spectrum analyzer 170. An incident optical signal present in optical fiber 110 is conveyed to output fiber 130 via coupler 120.
To monitor the input optical signal, an optical tap in coupler 120 extracts a small amount of signal power from optical fiber 110. Wavelength selective filter 140 selects a desired wavelength, and photodetector 150 converts the light associated with the selected wavelength into an electrical signal. The current from photodetector 150 is amplified by electrical amplifier 160, and subsequently measured by spectrum analyzer 170, or alternatively, an average value meter.
One of the signal characteristics measured by spectrum analyzer 170 is an estimated signal-to-noise ratio (SNR), which is commonly used to characterize a link performance in a WDM network. Optical domain spectral monitoring is typically used to estimate the SNR in optical networks. The signal is spectrally narrow, typically a few GHz. Assuming that the noise is slowly varying with wavelength, the optical noise level is measured at a wavelength slightly away from the channel""s signal, where the ratio of this noise to the optical signal represents the optical SNR.
Presently, an optical spectrum analyzer or an equivalent (e.g., a Hewlett Packard wavemeter) is used to measure the characteristics of an input optical signal in a WDM network. Such instruments, however, have a number of disadvantages. First, these instruments are expensive and slow (i.e., require scanning across all wavelengths). Second, there are well-known inaccuracies that result from such optical measurements. Third, some sources of signal noise are not detectable with these instruments. For example, interferometric intensity noise is one such source of signal noise, which is not detectable in the optical domain. Fourth, any induced jitter or wander cannot be detected using these instruments. Finally, the required optical SNR for low BER depends not only on the signal rate but also on the details of the receiver design.
Optical networks, such as WDM networks, can provide flexible broadband connectivity. A unique feature of WDM network technology is rate and format transparency. For example, the bit rate ƒbit of a signal may range from 25 Mb/s to 10 Gb/s. Furthermore, reconfigurable WDM networks perform highly variable route selection, a feature that is profoundly different from traditional point-to-point networks. At the periphery of such networks, any one set of users may employ at most a few line rates and formats. Within the network core, however, the full mix of rates and formats is encountered by most network elements. The route, line-rate, and format can be highly unpredictable and rapidly changing at any network element within a reconfigurable WDM network.
Like all other communication networks, it is desirable to monitor network transmission performance in WDM networks to anticipate problems before a user experiences poor service. Signal integrity and network link performance are closely monitored in traditional transmission systems, which operate at fixed line rates. These systems possess embedded signaling channels for diagnosis of transmission impairment and exchange of fault information.
For example, in Synchronous Optical Network (SONET) systems, the frame interval is continuously monitored along with verification of parity calculated on subsets of the bits within a frame. In these systems, loss of signal, loss of frame, and Bit Interleaved Parity 8 (BIP8) error rates are monitored and reported. Other transmission formats have their own embedded error detection.
Unlike traditional networks, however, WDM networks have less direct control over critical transmission parameters. Although many of the proposed WDM networks perform signal level management, they do not perform retiming, which is highly rate dependent and usually restrictive. When a WDM network performs little or no retiming, jitter management is delegated entirely to receivers at the user end. Accordingly, if the quality of the signal could be verified without full regeneration so that faults could be isolated to one subnetwork, it would facilitate implementation of multi-vendor interfaces for reconfigurable WDM networks.
Finally, interaction among protocol layers, such as locating faults when Internet Protocol (IP) routers or Asynchronous Transfer Mode (ATM) switches connect directly to optical networks, would be easier if a WDM network could dynamically determine the type of traffic it carries and test the quality of the traffic by checking for proper frame format and presence of errors. For example, when a network element in a WDM network detects an error in a signal, the network element could generate an alarm, facilitating the identification of the fault at the higher layer.
It is desirable to have a method and apparatus for performing automated time domain monitoring in optical networks that overcome the above and other disadvantages of the prior art. Methods and apparatuses consistent with the present invention determine characteristics of an input optical signal by estimating a minimum time interval between transitions in the input signal, determining a clock signal based on the estimated minimum time interval, and performing a time domain measurement on the input signal based on the determined clock signal.
In one embodiment, an optical signal monitoring apparatus comprises a forward rate detector, clock recovery circuit, and a time domain measurement circuit. The forward rate detector estimates the minimum time interval between transitions in an input optical signal. Based on the estimated minimum time interval, the clock recovery circuit extracts a clock signal from the input signal. Using the extracted clock signal, the time domain measurement circuit samples the input signal, and determines in time domain the characteristics of the input signal.
Methods and apparatuses consistent with the invention have several advantages over the prior art. In an optical network, many different time domain measurements may be performed on an optical signal once a clock is extracted. The clock is essential for measuring an eye-pattern, from which one can determine activity on a particular wavelength, directly calculate signal power levels and noise, and calculate jitter. With automated measurement, both instantaneous fault and average non-fault behavior may be used in fault isolation. The clock may be used to identify bit boundaries and to read the signal. Pattern detection may be used to recognize, for example, SONET A1A2 framing or the special symbols used in block coding. This would allow network elements to perform many of the conventional bit-level performance monitoring operations, even without benefit of information from network management.
One may determine the type of traffic present on a wavelength (if it is in a local catalog) and inform network management of the traffic type and quality. This would obviously be useful for preventing provisioning errors (by detecting that the traffic type present does not match the traffic expected) when jumpers are connected incorrectly. Also, this could provide a means for billing the customer, even though the customer has a transparent connection and can send a wide range of rates and types of traffic. It could also provide a near-real-time inventory of traffic statistics to network management. None of these things can be done today in the optical domain, and using inflexible fixed regeneration sacrifices transparencyxe2x80x94the regeneration circuit would need to be changed whenever the traffic type or speed is changed.
When signals cross administrative boundaries, for example between a local exchange carrier (LEC) and an inter exchange carrier (IEC) or between a private network and an LEC, it is necessary to determine that the signal is not impaired before it leaves one network and enters another. Today, this requires full regeneration and well-defined single-wavelength interfaces, such as SONET OC-3 or electrical DS-1 or DS-3. As bandwidth demands grow, it is desirable to eliminate this regeneration and permit transparent, multi-wavelength access by measuring the quality of the signal.